When creating a new field for profile you'll use the field's name: "purchase_details" (the label can be whatever you want) each user can edit this field when editing their profile and fill there the contact information that can be used for the other party to contact him (those can be bank data or any other information for the seller/buyer).
This field will be visible only for administrator or the author of the advert. So it's important to set the field to be visible like this.
Here's the screenshot of how the field should be set up:
Now, when user edit the profile - the new field is visible and ready to be filled (on the screenshot you'll see exemplary filling):
When filled, the author or administrator will see a button to trigger the popup with those info in their History of sales and orders:
If the field wont be filled by the user it'll display only username + email.